The events in the east were unfolding just hours after a Ukrainian military officer was shot and killed in Crimea in a confrontation with Russian troops.

Several hundred pro-Russian demonstrators who have seized government buildings in the city of Donetsk, in eastern Ukraine, urged President Vladimir V Putin on Monday to send troops to the region as a peacekeeping force, and they demanded a referendum on seceding from Ukraine and joining Russia.

The renewed unrest in eastern Ukraine, which flared on Sunday with coordinated demonstrations by thousands of pro-Russian protesters in Donetsk, Kharkiv and Luhansk, reignited fears in Kiev and the West about Russian military action a little more than a month after Russian forces occupied Crimea. The Kremlin annexed Crimea after a referendum there last month.

The events in the east were unfolding just hours after a Ukrainian military officer was shot and killed in Crimea in a confrontation with Russian troops.

A spokesman for the Ukrainian Defence Ministry, Vladislav Seleznev, said the officer, Maj Stanislav Karchevskiy, was killed in a military dormitory where he lived with his wife and two children, next to the Novofedorivka air base in western Crimea.

By about noon, the police in Donetsk said they were negotiating with representatives of about 150 protesters who had been occupying the regional administration building after breaking through a police cordon on Sunday.

Demonstrators said they had formed a new legislature and would move ahead with plans to hold a referendum on May 11, two weeks before the provisional Ukrainian government in Kiev is set to hold a national presidential election.

Several organizers of the protest in Donetsk spoke inside the regional administration building, where Russian television channels were broadcasting the events live.

In Germany, a spokesman for Chancellor Angela Merkel Monday said the government was extremely concerned about the events in eastern Ukraine and called for calm.

“The latest developments in Donetsk and Kharkiv are something which we are all very worried about in the German government,” the spokesman, Steffen Seibert, said at a news conference.

“We must urgently renew our appeal to all those in positions of responsibility to help stabilize the region and avoid such escalation,” he said.

The death of the Ukrainian officer was a rare instance of deadly violence as Ukrainian forces continue their withdrawal from the peninsula after its annexation by Russia.